Such facilities are very well known. Substrates to be coated or to be densified by pyrolytic carbon are placed in a furnace into which a reactive gas containing one or more precursors of the carbon is introduced. The precursor gas is a hydrocarbon, typically methane, propane or a mixture of the two. The pressure and the temperature in the furnace are adjusted in order to produce the pyrolytic carbon coating or matrix by decomposition (cracking) of the precursor gas in contact with the substrates. The effluent gas containing reaction by-products is removed from the furnace by continuous pumping.
The reaction by-products comprise organic compounds that have a relatively high solidification temperature, in particular polycyclic aromatic hydrocarbons (PAHs) such as, notably, naphthalene, pyrene, anthracene, acenaphthylene, etc. A list of the PAHs is indicated in the table below:
PAHCAS No.Naphthalene91-2062Acenaphthylene208-96-8Acenaphthalene83-32-9Fluorene86-73-7Anthracene120-12-7Phenanthrene85-01-08Fluoranthene206-44-0Pyrene129-00-0Benzo(a)anthracene56-55-3Chrysene218-01-9Benzo(a)pyrene50-32-8Benzo(b)fluoranthene205-99-2Dibenzo(ah)anthracene53-70-3Benzo(k)fluoranthene207-08-9Benzo(ghi)perylene191-24-2Indeno(123cd)pyrene193-39-5
By condensing, these reaction by-products form tars which have a tendency to be deposited in the outlet lines of the furnace as the effluent gas cools. These tars are also found in the pumping device, for example in the oil of vacuum pumps or in the condensates of steam ejectors.
Similar problems may be encountered with industrial facilities other than chemical vapor infiltration or deposition furnaces that use hydrocarbons as reactive gas, for example carburizing furnaces.
It is known from application WO 03/047725 to use an aromatic mineral oil to treat the effluent gas containing tars and in particular polycyclic aromatic hydrocarbons (PAHs) by a step of spray scrubbing with this oil. The trapping of the PAHs is followed by raising the level in the tar trap. From a certain threshold of captured PAHs, the PAH-loaded oil is removed in order to be replaced by fresh oil; the current threshold is at around 30% uptake by volume. By continuing the use of the oil we have shown that the trapping efficiency is maintained up to more than 60% by volume. Unfortunately, even though the trapping may be maintained up to these levels, the physicochemical characteristics (density, viscosity) of the PAH-loaded oil change over time as a function of its PAH content. In particular, its viscosity will increase to unacceptable values that will prevent the pumping and spraying thereof. The utilization time of this aromatic mineral oil is therefore limited, which makes it necessary to replace it and therefore renders the process expensive.
Patent application WO 2010/085244 describes the use of food oil for eliminating PAHs from gas. However, this type of oil has the disadvantage of degrading thermally. Thus, it is necessary to cool the gas before scrubbing it with the oil so as to obtain a scrubbing temperature below 80° C. and even below 50° C. ([0023] page 7). Naphthenic oils are cited in table 1 on page 14 of this document but are not considered to be of interest due to the low solubility level of the PAHs in these oils.
Application U.S. Pat. No. 5,019,143 describes the use of various solvents selected from the group consisting of paraffinic solvents, naphthenic solvents, aromatic compounds or other solvents for extracting ethylene from a gas that also contains heavier unsaturated and saturated hydrocarbons. However, the naphthenic solvents cited are not naphthenic oils. In addition, this document does not encourage the mixing of the various types of solvents. Furthermore, this document does not at any time indicate that the gas to be treated contains PAHs. Finally, the operating pressure of the process is very high (between 50 and 400 psi, i.e. between 0.34 MPa and 2.76 MPa).